Ultrasonic transducers and imaging systems are used in many medical applications and, 10 in particular, for the non-invasive acquisition of images of organs and conditions within a patient, typical examples being the ultrasound imaging of fetuses and the heart. Such systems commonly use a linear or phased array transducer having multiple transmitting and receiving elements to transmit and receive narrowly focused and "steerable" beams, or "lines", of ultrasonic energy into and from the body. The transmitted beams, or lines, are reflected from the body's internal structures as received beams, or lines, that contain information that is used to generate images of the body's internal structures.
In a typical application, such as cardiac scanning, a number of beams or lines are transmitted and received along a plurality of angles forming a sector, that is, a wedge shaped three dimensional volume of interest, wherein the angular width of a sector may be the full range of angles that the transducer is capable of generating and receiving, or a selected a portion of that range. The lines of a sector are typically then organized into "frames" wherein each frame contains data representing a volume of interest, that is, a sector, and may be further processed or viewed to extract or present the information of interest.
The sequence and timing in which the lines are acquired and the organization of lines into frames often depends upon the particular application and the information desired and is affected by such factors as the dynamics of the information that is being acquired, the time required to transmit and receive a line, the data processing necessary to extract the desired information, and the processing required to generate an image display of the information. For example, in certain types of cardiac scanning the frames may be organized so that each frame contains data representing the sector at a selected point in time in the cardiac cycle so that the dynamic operation of the heart in a volume of interest can then be observed by viewing successive frames.
In many applications, however, these requirements conflict or interact to product undesirable results. For example, one important application of ultrasonic imaging is color flow mapping wherein doppler information is extracted from the returning signals to generate images, or maps, of blood flow velocity in, for example, the chambers of a heart. Color flow mapping, however, requires multiple data acquisitions, typically 8 to 12 along each line, and the time required for each acquisition along a line is determined by the speed of ultrasound wave in the body and the maximum depth of the volume of interest from the transducer. As a result, one or all of the frame rate, that is, the rate at which data is acquired, the line density, that is, the granularity or sharpness of the map as determined by the number of lines used to generate the map, or the field of view, that is, the angular width and depth of the sector as determined by the number and length of the lines, are compromised.
The ultrasound imaging systems of the prior art, including color flow mapping systems, have addressed this problem in a number of ways, such as allowing the systems to be configured in operate in either or both of the "rapid burst" and "interleaved line" modes. In the "rapid burst" mode, the system transmits and receives a sequence or set of lines along a single direction, wherein the set of lines along a single direction is referred to as a "packet", and this process is repeated across the sector so that the set of all of the packets of the sector comprise a frame. This approach is advantageous in applications where the condition being observed is changing rapidly, such as in blood flow mapping of regions wherein the blood is fast flowing or wherein it is necessary to identify and map relatively short transients in the blood flow. In this instance, a frame represents a relatively small interval in time with respect to the cardiac cycle, but is sufficient in time to show blood movement.
In the "interleaved line mode" the system interleaves the acquisition of lines in a pattern among a sequence of two or more frames. This approach may be used, for example, where the condition being observed is relatively stable or repetitive over time, such as blood flow mapping in regions where blood is moving relatively slowly, so that longer sampling times are necessary in order to observe blood movement, or where it is not necessary to detect transient conditions. Because of the longer time required to observe a change in the observed condition, and thereby in order to save scanning time, a system will often not scan a full frame of lines in the interleaved line mode but instead will scan a selected sub-set of the lines comprising a frame of interest. This approach may also be used or where the flow of blood is too fast to capture using the "rapid burst" method, but repeats periodically with the cardiac cycle.
A significant limitation of systems requiring multiple acquisitions along each line, however, such as blood flow mapping systems or B-mode systems, is that the requirement for multiple acquisitions along each line limits the number of lines that can be acquired in an allowable time, even using "rapid burst" or "interleaved line" operation, thereby limiting the data acquisition rate of the system and, for example, the system resolution.
A recurring problem in ultrasonic imaging systems, therefore, is that many ultrasonic imaging applications have data acquisition requirements that may significantly compromise any of all of the frame rate, that is, the rate at which data is acquired, the line density, that is, the granularity or sharpness of the map, the field of view, that is, the angular width and depth of the sector, or the rate at which data may be presented to a viewer. One example that has been discussed above is color flow imaging, which requires multiple data acquisitions along each are enhanced by increases in the data acquisition rate. Color flow mapping as discussed above, for example, requires multiple data acquisitions along each line and, as also discussed above, the performance of a color flow mapping system can be significantly enhanced by an increase in the data acquisition rate. Yet another example is three dimensional (3D) imaging, which is typically performed by acquiring and storing two dimensional tomographic image slices to construct a body of data representing a three dimensional image that is subsequently processed to provide images along any plane through the imaged space. A typical 3D imaging system, for example, acquires 30 tomographic slices, each 60.degree. wide and 16 cm deep, in order to generate a three dimension image and requires about 8 milliseconds to acquire each frame, giving approximately a 4 Hz update date. It is therefore apparent, that the implementation of a real time three dimensional imaging system would require a significant increase in the data acquisition rate.
As discussed above, the prior art has applied several methods for increasing the data acquisition rate in an ultrasound system, including the use multiple parallel receiving lines, multiple transmissions and non-consecutive scanning, in order to increase the data acquisition rate of an imaging system. It may be seen, however, that the above described techniques for increasing the data acquisition rate are ultimately limited by the number of simultaneous distinct transmission and reception lines that can be formed by a transducer without unresolvable or uncorrectable mutual interference between the lines or degradation of the data and the resulting image from, for example, spatial artifacts.
The present invention provides a solution to these and other problems of the prior art by increasing the number of transmission and reception lines that can be formed by a transducer.